1. Field of the Invention
The present invention relates to current-perpendicular-to-plane (CPP) magnetic sensing elements and, more particularly, to a magnetic sensing element in which the rate of change in resistance (ΔR/R) can be effectively improved.
2. Description of the Related Art
FIG. 8 is a partial sectional view of a conventional magnetic sensing element, viewed from the surface facing a recording medium.
The magnetic sensing element shown in FIG. 8 includes a lower electrode 1, an antiferromagnetic layer 2 composed of PtMn or the like formed on the lower electrode 1, a pinned magnetic layer 3 composed of CoFe or the like formed on the antiferromagnetic layer 2, a nonmagnetic material layer 4 composed of Cu or the like formed on the pinned magnetic layer 3, a free magnetic layer 5 composed of NiFe or the like formed on the nonmagnetic material layer 4, and an upper electrode 6 formed on the free magnetic layer 5.
As shown in FIG. 8, the magnetization of the pinned magnetic layer 3 is pinned in the Y direction by an exchange anisotropic magnetic field with the antiferromagnetic layer 2.
The magnetization of the free magnetic layer 5 is aligned in the X direction by a longitudinal bias magnetic field from hard bias layers (not shown in the drawing) formed on both sides in the track width direction (in the X direction) of the free magnetic layer 5.
The magnetic sensing element shown in FIG. 8 is a so-called current-perpendicular-to-plane (CPP) magnetic sensing element in which a current from one of the electrodes 1 and 6 flows through the multilayer film including the antiferromagnetic layer 2 to the free magnetic layer 5 in the thickness direction (in the Z direction).
In the CPP magnetic sensing element, read output can be increased due to the reduction in the element size compared to a current-in-plane (CIP) magnetic sensing element in which a current from the electrode flows parallel to the constituent layers of the multilayer film, and the CPP type is expected to be suitable for decreasing of the element size in connection with increasing recording densities.
One of the problems to be solved in order to implement practical use of CPP magnetic sensing elements in view of increasing recording densities is an improvement in the rate of change in resistance (ΔR/R). In order to improve the rate of change in resistance, a change in resistance (ΔR) must be improved.
It is known that the change in resistance (ΔR) is proportional to [β2/(1−β2)]·ρF·tF, where β is the value determined by the materials for the ferromagnetic layers (pinned magnetic layer 3 and free magnetic layer 5), and the relationship ρ⇓/ρ⇑=(1+β)/(1−β) is satisfied, where ρ⇓ is the resistivity to spin-down conduction electrons and ρ⇑ is the resistivity to spin-up conduction electrons. ρF is the resistivity of the ferromagnetic layer (the average of resistivity to spin-down conduction electrons and resistivity to spin-up conduction electrons) and tF is the thickness of the ferromagnetic layer.
As the ferromagnetic layer, a CoFe alloy or the like has been used. In the CoFe alloy, β is about 0.5, and the resistivity ρF is about 16 μΩ·cm.
In “How predictable is the current perpendicular to plane magnetoresistance? (invited)”, J. Appl. Phys. 79(8), Apr. 15, 1996, Table 1 discloses that the value β of′ Co is 0.38 to 0.54 and the value β of Ni84Fe16 is 0.34 to 0.66.
In “Andreev reflection: A new means to determine the spin polarization of ferromagnetic materials”, 1999 American Institute of Physics, Graph 4 discloses that the polarizability P of each of NiFe, Co, Ni, and Fe is about 0.33 to 0.45. It is known that the “polarizability P” is correlated with the value β, and the value β (absolute value) increases as the polarizability P increases.
However, with respect to magnetic materials, such as CoFe alloys, Co, Ni84Fe16, NiFe, Ni, and Fe, the value β and the polarizability P are not sufficiently large. In order to cope with higher recording densities, an improvement in the rate of change in resistance (ΔR/R) is expected by further increasing the change in resistance (ΔR).